An Introduction to 700 Million Years of Earth History in Shropshire and Herefordshire

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An introduction to 700 million years of earth history in Shropshire and Herefordshire

Peter Toghill1

TOGHILL, P. (2008). An introduction to 700 million years of earth history in Shropshire and Herefordshire. Proceedings of the Shropshire Geological Society, 13, 8–24. The beautiful landscape of the Welsh Marches is underlain by a rock sequence representing 10 of the 12 recognised periods of geological time. This remarkable variety, covering 700 million years of Earth history, has resulted from the interplay of three main factors: (1) erosion and faulting which have produced a very complex outcrop pattern; (2) southern Britain's position near to plate boundaries through most of late Precambrian and Phanerozoic time; and, most importantly, (3) the incredible 12,000 km, 500 million year, journey of southern Britain across the Earth's surface from the southern hemisphere to the northern, caused by plate tectonic processes.

1Church Stretton, Shropshire, UK. E-mail: [email protected]

BACKGROUND This paper set the geological scene for the one-day symposium at Ludlow forming the centrepiece of the 2007 Marches Festival of Geology. However, this is not the place to provide a detailed description of the geology of Shropshire per se, for which the reader is referred to Peter Toghill’s Geology of Shropshire (2006), thereby to benefit from the author’s detailed knowledge of the local geology. The beautiful landscape of the Welsh Marches is underlain by a rock sequence representing ten of the twelve recognised periods of geological time (10 out of 13 if the Tertiary is subdivided into two periods). This remarkable variety, covering 700 million years of Earth history, has resulted from the interplay of three main factors: (1) erosion and faulting which have produced a very complex outcrop pattern; (2) southern Britain's position near to plate boundaries through most of late Precambrian and Phanerozoic time; and, most importantly, (3) the incredible 12,000 km, 500 million year, journey of southern Britain across the Figure 1. Geological map of Shropshire. Only the major Earth's surface from the southern hemisphere to faults are shown. Most information based on British the northern, caused by plate tectonic processes Geological Survey 1:250,000 Series Map, Solid Geology, (Figures 1, 2, 3 & 4). Mid-Wales and Marches, 1990, by permission of the British Geological Survey, copyright permit IPR/51-03C.

INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE

shows Shropshire’s incredible journey of 12,000 km in 500 million years. Redrawn with permission from data in Woodcock and Strachan, Geological History of Britain and Ireland, Blackwell Science, 2000, p.30, Fig. 2.7. Chapter 3.

Figure 2. Structural Geology of Shropshire. Most information from British Geological Survey 1:250,000 Series Map, Solid Geology, Mid-Wales and Marches, 1990, by permission of the British Geological Survey, copyright permit IPR/51-03C.

Figure 3. Movement through latitudes of the continents and micro continents that included parts of Britain, from late Precambrian to the present day. The orange line

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The story begins on the northern margins of years ago (Ma) to form a marginal basin bounded Gondwana, around 60 degrees south of the equator by faults now called the Welsh Borderland Fault ca. 600 million years ago (Figures 5 & 6). System. The most famous of these faults, the Church Stretton Fault, together with its partner further west, the Pontesford-Linley fault, would have a profound affect on Shropshire geology for nearly 500 million years (Figure 7).

Figure 5. Late Precambrian palaeogeography. (a) Vendian supercontinent showing positions of Cadomian Volcanic Arc and continental split from Iapetus Ocean; (b) formation of early Iapetus Ocean. Redrawn with permission from data in Woodcock and Strachan, Geological History of Britain and Ireland, Blackwell Science, 2000, p.25 and 26, Figs. 2.4b and 2.5a.

Figure 7. Late Precambrian Welsh Borderland Marginal Basin showing formation of Uriconian Volcanics and Longmyndian Supergroup. The late Uriconian and early Longmyndian (Stretton Group) are probably of the same age. © Copyright 2006 Peter Toghill

A volcanic arc here, near to the margins of the basin, formed the Uriconian Volcanics of Shropshire between 570 and 560 Ma, and around the same time a nearby shallow marine basin received sediments from the eroding volcanic arc and adjacent areas to form the unique Longmyndian Supergroup of sedimentary rocks, up to 7,000 m thick. Figure 6. Late Precambrian plate tectonics. Formation of The marine basin rapidly shallowed so that the Rushton Schists and Primrose Hill Gneisses and Schists, and formation of pull-apart marginal basin bounded by early Longmyndian fine grained marine and the Welsh Borderland Fault System including Church deltaic/fluvial sequence, the Stretton Group, is Stretton and Pontesford-Linley Faults. © Copyright 2006 followed, without an unconformity, by the coarse Peter Toghill grained fluvial Wentnor Group. Major earth movements around 550 Ma (latest A late Precambrian basement of metamorphic Precambrian) formed the remarkable overturned rocks and igneous complexes, now exposed as the Longmynd Syncline, exposed around Church Rushton Schists of Shropshire and the igneous Stretton between the Church Stretton and rocks of the Hanter and Stanner Hills in Pontesford-Linley Faults, with small exposures Herefordshire, was split apart around 600 million further south around Old Radnor. The compression

INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE was mainly transpressional, with considerable lateral movements along the Pontesford-Linley Fault and Church Stretton Fault, and it is possible that the Longmyndian is a displaced terrane (Figures 8 to 22).

Figure 11. Rushton Schist poorly exposed in a rutted track. Wrekin in the background (long ridge); Little (Primrose) Hill is the small “bump” to its right. © Figure 9. Long Mynd Syncline approximately along the Copyright 2006 Peter Toghill line A-B in Figure 10. Present-day relationships of Longmyndian and Uriconian. PLF, Pontesford-Linley Fault; RC, Radlith Conglomerate; OC, Oakswood Conglomerate; LC, Lawnhill Conglomerate; SC, Stanbatch Conglomerate; DC, Darnford Conglomerate; HC, Haughmond Conglomerate; HUC, Huckster Conglomerate; BV, Batch Volcanics; CG, Cardingmill Grit; BR, Buxton Rock; HG, Helmeth Grit; CSF, Church Stretton Fault. © Copyright 2006 Peter Toghill

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Figure 12. Church Stretton Valley from the Long Mynd Figure 15. Finely laminated, water-lain, Ragleth Tuffs on looking towards Caer Caradoc and the Lawley, with the the flanks of Caer Caradoc. © Copyright 2006 Peter Wrekin in the distance. The main Church Stretton Fault is Toghill one-third of the way up the side of Caer Caradoc. Church Stretton Valley has only one main fault along it and is not a rift valley. This fault could be a listric fault. © Copyright 2006 Peter Toghill

Figure 16. The Battlestones. A rhyolitic crag at the northern end of Willstone Hill with Willstone Hill Conglomerates (? Wentnor Group Longmyndian) to the left of the crag. Caer Caradoc in the distance. © Figure 13. In the foreground: Uriconian andesites on the Copyright 2006 Peter Toghill Lawley; looking south towards Caer Caradoc and the Long Mynd (beyond). Despite the conical shape of Caer Caradoc, it is not a volcanic cone but a remnant of a wider extent of Uriconian lavas and ashes – no vents have been found within the Uriconian rocks of Shropshire. © Copyright 2006 Peter Toghill

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Figure 20. Cleaved purple shales in the Synalds Formation, Longmyndian, Haddon Hill, Long Mynd. The beds are dipping steeply to the left whereas the cleavage is just about vertical, parallel to the ski stick. © Copyright 2006 Peter Toghill

Figure 18. The Longmyndian sequence showing environments of deposition and types of sediment. After Pauley, 1990, Sedimentology, Structural Evolution and Tectonic Setting of the Late Precambrian Longmyndian Supergroup of the Welsh Borderland, p.345, Fig.3. In D’Leoms et al., 1990, The Cadomian Orogeny. Figure 21. Possible fossil rain prints? Synalds Formation, Geological Society Special Publication No.51, pp. 341- Longmyndian. © Copyright 2006 Peter Toghill 351.

Figure 22. Darnford Conglomerate in the Bayston- Figure 19. Burway Formation turbidites, Burway Hill, Oakswood Formation, Wentnor Group, Longmyndian. Long Mynd. © Copyright 2006 Peter Toghill Sharpstones Quarry, Bayston Hill. Finger is pointing at

A major marine transgression following further widening of the Iapetus Ocean marked the start of the Cambrian period (545 Ma to 495 Ma) (Figures 23 to 26), with a major unconformity below shallow water quartzites and sandstones which formed over the Welsh Marches, now exposed around the Wrekin and Church Stretton. These

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Cambrian rocks yielded Britain's oldest trilobites in the late 1800s.

Figure 23. Late Cambrian palaeogeography (ca. 500 Ma) showing ancient latitudes of Scotland, England and Figure 26. The Comley area below Caer Caradoc, near Wales, and positions of trilobite faunal provinces. Below: Church Stretton. Comley Quarry is just visible in woods cross-section through Iapetus Ocean at its widest extent immediately behind the farm in the centre. © Copyright during the late Cambrian. © Copyright 2006 Peter Toghill 2006 Peter Toghill

During the Ordovician (495 to 443 Ma) the Iapetus Ocean started to close as the Avalonian microcontinent split away from Gondwana and moved northward, "pushed" by the spreading Rheic ocean to the south. By the end of the Ordovician southern Britain had moved from 60 degrees south to 30 degrees south of the equator. The Welsh Marches were right astride the Iapetus southern shoreline, which fluctuated west to east to form very different Figure 24. Unconformity (strictly: a non-conformity) at sequences either side of the Pontesford-Linley Ercall Quarries. Cambrian Wrekin Quartzite dipping fault. steeply south-east (to the right) rests unconformably on An early Ordovician (Tremadoc) mud blanket the pink late Precambrian Ercall Granophyre. © encroached over the whole area followed by a Copyright 2006 Peter Toghill rapid regression of the sea west of the Pontesford- Linley fault, so that Arenig to Llanvirn epoch sequences, amounting to 4,000 m including the famous Stiperstones Quarztite and volcanic lavas and ashes, only occur west of the fault. (Figures 27 to 37).

INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE

Figure 28. Middle Ordovician (Caradoc Epoch) palaeogeography. Above: rapid closure of the Iapetus Ocean and growth of the Rheic Ocean. Below: cross- Figure 27. Early Ordovician (Arenig Epoch) section through the Iapetus Ocean during the Caradoc palaeogeography. Above: early stages of Iapetus Ocean Epoch. Data from Trench & Torsvik, 1992, Journal of the closure and opening of the Rheic Ocean. Below: cross- Geological Society of London, 149, Fig. 2. section through the Iapetus Ocean during the Arenig

Epoch. Data from Trench & Torsvik, 1992, Journal of the Geological Society of London, 149, Fig. 1.

Figure 29. Late Ordovician (Ashgill Epoch) palaeogeography. Above: collision of Avalonia and Baltica, and Gondwana glaciation. Below: cross-section through the Iapetus Ocean during the Ashgill Epoch. Data from Trench & Torsvik, 1992, Journal of the Geological Society of London, 149, Fig. 3.

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Figure 31. Contrasting Ordovician sequences in the Shelve and Caradoc areas, west and east of the Pontesford-Linley Fault. Pontesford area also shown. WVF, Whittery Volcanics Formation; WSF, Whittery Shale Formation; HASF, Hagley Shale Formation; HAVF, Hagley Volcanics Formation; ASF, Aldress Shale Formation; SWSF, Spywood Sandstone Formation; RSF, Rorrington Shales Formation; MF, Meadowtown Formation; BSF, Betton Shales Formation; WFF, Weston Flags Formation; HSF, Hope Shale Formation; SVM, Stapeley Volcanics Member; HVM, Hyssington Volcanics Member; MFF, Mytton Flags Formation; SQF, Stiperstones Quartzite Formation; SHSF, Shineton Shales Formation; LO, Longmyndian; U, Uriconian; POS, Pontesford Shales; OS, Onny Shales; ASG, Acton Scott Group; CLF, Cheney Longville Flags; AL, alternata Limestones; CS, Chatwall Sandstone; HS, Harnage Shales; SWB, Smeathen Wood Beds; HEG, Hoard Edge Grit; CB, Coston Beds; TR, Tremadoc; CA, Cambrian. © Copyright 2006 Peter Toghill

INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE

Figure 32. Ordovician transgressions and regressions across the Pontesford-Linley and Church Stretton Faults. © Copyright 2006 Peter Toghill Figure 34. Callow Quarry. Close-up of Mytton Flags showing steep dip produced by late Ordovician (Shelvian) folding. © Copyright 2006 Peter Toghill

Figure 35. Shelveian folding in southern Shelve Inlier showing Shelve Anticline and Llan (Ritton Castle) Syncline, and Corndon phacolith. WV, Weston Formation; SV, Stapeley Volcanics; A, Andesite; D, Figure 33. Ice Age tors on the Stiperstones. The Dolerite; HV, Hyssington Volcanics; HS, Hope Shales; Stiperstones Quartzite at Cranberry Rock looking north MF, Mytton Flags; SQ, Stiperstones Quartzite; SS, towards Manstone Rock, the highest point on the ridge, Shineton Shales; SF, Stiperstones Fault; PLF, Pontesford- and Shropshire’s second highest hill at 537 m AOD. © Linley Fault; V, Uriconian Volcanics; L, Longmyndian. Copyright 2006 Peter Toghill © Copyright 2006 Peter Toghill

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The Silurian period (443 to 418 Ma) made famous by Murchison saw the Welsh Marches in the southern tropics, around 25 to 20 degrees south (Figures 38 to 44).

Figure 38. Final closure of the Iapetus Ocean during the late Silurian Wenlock Epoch (ca. 425 Ma). By the end of the Silurian period, the Iapetus Ocean had disappeared completely and the Caledonian Mountains formed along the suture line between Laurentia to the west, and Baltica and Avalonia to the east. Data from Trench & Torsvik, 1992, Journal of the Geological Society of London, 149, p. 869, Fig. 4.

Figure 37. “Murchison’s View”: escarpments in the vicinity of Caer Caradoc – a view featured in Murchison’s Silurian System (p. 216), from the slopes of Caer Caradoc looking north towards the Lawley and the Ordovician scarps of Hoar Edge and Yell Bank. © Copyright 2006 Peter Toghill

The well known Caradoc epoch transgression spread shallow water sequences east of the Pontesford-Linley fault in the type area around Church Stretton. A major regression in the Ashgill epoch caused by tectonic uplift, the Shelveian event, and amplified by falls in sea level caused by a northern Gondwana glaciation, meant that the sea retreated west into Wales during the late Ordovician. Major folding, faulting and igneous activity during the Ashgill Shelveian event affected the Shelve area in particular. Much of the faulting was horizontal tear faulting with considerable displacements possible along the Pontesford- Linley fault which may be a terrane boundary. The Shelve area may have been joined with Builth area Figure 39. Distribution of brachiopod communities during the Ordovician and then displaced north during the late Llandovery Epoch. After Ziegler et al., during the Shelveian event. Nature, 207, 270-272.

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Figure 41. Cross-section through the Ludlow Anticline south-west of Ludlow. Channels at the base of the Leintwardine Formation are shown in the north-west of the area. WOL, Woolhope Limestone; CF, Coalbrookdale Formation; MWL, Much Wenlock Limestone Formation; EF, Elton Formation; BF, Bringewood Formation; AL, Aymestry Limestone; Leintwardine Formation; WF, Whitcliffe Formation; LBB, Ludlow Bone Bed; DCS, Downton Castle Sandstone; TSF, Temeside Shale Formation; RG, Raglan Mudstone Formation. © Copyright 2006 Peter Toghill Figure 44. Ludlow Bone Bed (along the prominent notch) at Ludford Corner, Ludlow. © Copyright 2006 Peter Toghill

An early Silurian Llandovery transgression from the west (mapped by Ziegler, Cocks and McKerrow using brachiopod communities as depth indicators) produced shallow water sequences. This was followed by shallow water subtropical limestones with reefs (including the Figure 42. Changes in sedimentary sequences from shlf Much Wenlock Limestone), and shales, of the to basin in the Welsh Borders during the Silurian. The Wenlock and Ludlow epochs, e.g. around the section shows the facies changes and disappearance of southern end of the Long Mynd. At the top of the limestones from east to west across the Church Stretton Silurian it is deep-water graptolite faunas that have Fault. After Dineley, 1960, Field Studies, 1, Fig. 9. changed the international stratigraphic picture in recent decades. Murchison's neat Ludlow Bone Bed marker horizon did not fit with the graptolite-bearing type sections in the Pozáry Section of the Daleje Valley near Prague (Czech Republic), with the result that a new Prídolí Series now extends the Silurian upwards into our Old Red Sandstone lithologies, finishing at the Psammosteus Limestone (now the Bishops Frome Limestone Formation). At the end of the Silurian the Iapetus Ocean had almost closed, with southern and northern Britain

P. TOGHILL finally joined by the time of the mid Devonian. The Acadian (Late Caledonian) orogeny, represented by the unconformity between the Lower and Upper Old Red Sandstone, was responsible for the folding that gave us the Ludlow Anticline and the dip of the Wenlock Limestone, giving rise to our most conspicuous topographical feature: Wenlock Edge. During the Devonian period (418 to 362 Ma) non-marine Old Red Sandstone sediments here formed over the Welsh Marches (Figures 45 & 46). During the Carboniferous (362 to 290 Ma) periodic episodes of the Variscan Orogeny left their mark on a landscape which was now more subdued, and divided into a number of basins of deposition, but again with Shropshire in a crucial marginal position astride St George's Land, and right on the Equator (Figures 47 to 52).

Figure 47. World palaeogeography for the Devonian, Carboniferous and Permian, showing closure of the Rheic Ocean. Redrawn using data from the Open University, Course S236, Block 6, Historical Geology, p. 8, Fig. 3.

Figure 46. Devonian palaeogeography. Redrawn using data from the Open University, Course S236, Block 6, Historical Geology, p. 39, Fig. 24.

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Figure 51. Titterstone Clee Hill summit. View from the information board close to the Dhustone Quarry viewpoint. Large quarries in the vicinity are evidence of the huge amount of stone extracted hereabouts since the mid nineteenth century. © Copyright 2006 Peter Toghill

Figure 48. Early Carboniferous palaeogeography. Redrawn using data from the Open University, Course S236, Block 6, Historical Geology, p. 43, Fig. 29.

Figure 52. Brown Clee Hill from the summit of Titterstone Clee Hill. Blocks of dolerite (“Dhustone”) are scattered across the foreground. Basal Coal Measures form the bench on the west side (left and centre) of Brown Clee Hill. © Copyright 2006 Peter Toghill Figure 49. Carboniferous Limestone at Llanymynech Hill, Asbian Stage, Whitehaven Formation. © Copyright The result is a series of minor unconformities at 2006 Peter Toghill the base of the Carboniferous Limestone, the Namurian and the Coal Measures. This is well seen in the Clee Hills, where Titterstone Clee shows each of these Carboniferous Series, but in Brown Clee the Coal Measures rest directly on Old Red Sandstone. In the East Shropshire Coalfield the Upper Coal Measures spread unconformably over an earlier Variscan landscape with a break that earlier geologists referred to as the Symon Fault. Arid conditions set in during the late Carboniferous as Britain found itself in the arid

Figure 50. Folded and faulted Middle Coal Measures heart of Pangaea, just north of the Equator (Figure with coal seams. Opencast site, Telford. © Copyright 53). Desert sandstones covered the area during the 2006 Peter Toghill Permian and Triassic periods (290 to 206 Ma). During the late Triassic and the beginning of the

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Jurassic period marine conditions, with Britain around 35 degrees north, spread from the east to deposit clays and thin limestones. The early Jurassic Lias (ca. 200 Ma) is the youngest bedrock deposit now preserved in the Welsh Marches, around Prees in north Shropshire. We have no record of younger Jurassic or Cretaceous rocks but it is likely that Middle Jurassic rocks and the Chalk were deposited but have since been removed by erosion (Figures 54 to 59).

Figure 55. Grinshill Quarry, a principal source of high quality building stone. Massive, creamy brown sandstone, Grinshill Sandstone, with some cross-bedding overlain by flaggy Tarporley Siltstones (“Waterstones”) and red Mercia Mudstones at the very top of the quarry face. © Copyright 2006 Peter Toghill

Figure 57. Early Jurassic palaeogeography. “P” is the Prees outlier, in north-east Shropshire. Redrawn using data from the Open University, Course S236, Block 6, Historical Geology, p. 59, Fig. 42.

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Figure 61. Tertiary basalt dyke in Grinshill Quarry. The Figure 59. Late Cretaceous palaeogeography showing dyke is 0.8 m wide. Fresh “blue” basalt can be seen shorelines thought to exist at the time of the deposition of surrounded by reddish brown lateritic weathered dyke the chalk. © Copyright 2006 Peter Toghill material. © Copyright 2006 Peter Toghill

During the Tertiary period (65 to 2 Ma) the Welsh Marches, now close to their present latitude, experienced uplift ( with dyke intrusions in the north) and erosion of great thicknesses of Mesozoic rocks, to expose a landscape not too dissimilar to today, but which was extensively modified during the Quaternary Ice Ages and post glacial periods (2Ma to present day). Although little primary glacial erosion took place, subglacial channels and proglacial lakes enabled river diversions (Figures 60 to 64).

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ACKNOWLEDGEMENTS This paper has been compiled by Michael Rosenbaum and checked by the author from notes taken at his lecture in Ludlow on 13th September, 2007, as a part of the Marches Figure 62. Devensian ice sheets in Shropshire. © Festival of Geology. Copyright 2006 Peter Toghill

REFERENCES

For a comprehensive list the reader is referred to the references cited in: Toghill, P. (2006). Geology of Shropshire. 2nd Edition, The Crowood Press, Marlborough, 256 pp.